Technical Abstract:
Introduction: Biochar is a charcoal-like material generated during the pyrolysis of biomass. Because of its high carbon (C) content and its recalcitrant nature there has been much interest in using biochar to sequester C from the atmosphere by incorporating it into soils. Research has also indicated that biochar addition to soils can help address a number of environmental and agricultural problems such as improving soil physical and chemical properties, increasing plant-available soil water, and reducing environmental pollution by effectively retaining nutrients, heavy metals, and pesticides in the soil. Knowledge concerning the impact of biochar addition on the retention and transport of microorganisms, however, is nonexistent.
Methods: The goal of this study was to evaluate whether biochar amendments affect the transport of E. coli – one of three indicator organisms used by EPA to determine whether a groundwater source has been fecally contaminated – through a sandy soil. Our study hypothesis is that the transport of E. coli through biochar-amended soils will vary depending on the feedstock composition and pyrolysis temperature of the biochar as both of these factors strongly influence the physical and chemical properties of biochar. To address our hypothesis we looked at the transport of two E. coli isolates, which varied in their surface charge, through 10-cm columns packed with fine sand amended under both water-saturated and unsaturated conditions. Biochar treatments included two different feedstock (poultry litter and pine chips) and biochar pyrolyzed at two different temperatures (350 and 700 o C).
Results: The addition of biochar to the fine sand resulted in significant changes to soil pH, total carbon content of the soil, and the soil cation exchange capacity. Biochar addition to the soil also increased pH and DOC concentrations in the column effluent and significantly modified the ionic composition of the soil water. Changes in soil and soil solution properties were dependent on biochar feedstock and pyrolysis temperature of the biochar. Specifically, the high temperature biochars resulted in greater pH increases whereas the low temperature biochars produced greater increases in DOC concentrations.
The effect of biochar-amendments on the transport behavior of E. coli through the sandy soil varied depending on biochar application rate, biochar feedstock composition, biochar pyrolysis temperature, soil moisture condition, and the surface properties of the E. coli isolates. Addition of the low temperature poultry litter biochar increased E. coli transport whereas the high temperature poultry litter biochar generally decreased E. coli transport. Biochar produced from pine chips, on the other hand, decreased transport of E. coli regardless of pyrolysis temperature, though the high temperature biochar was much more effective in decreasing transport. Biochar addition had a greater affect on the transport behavior of the E. coli isolate with a low net negative charge than the isolate with a high net negative charge indicating the important role bacterial surface properties play on interactions between biochar and soil microorganisms.
Conclusions: Our results indicate that some biochars may be effective in reducing the leaching of microorganisms through soil and provide important information into the potential mechanisms affecting the retention and transport of E. coli through biochar-amended soils.